In recent years semiconductor Hall elements have been used for electronic measurements of electric power. The feeding current is proportional to the mains voltage and the magnetic induction is proportional to the mains current, hence the Hall voltage is proportional to the power. The Hall voltage expression is: EQU U.sub.H =K(T).multidot.I.sub.N .multidot.B=K(T).multidot.k.sub.1 U.multidot.k.sub.2 I
with the following meanings of the designations: Hall voltage U.sub.H, feeding current I.sub.N, magnetic induction B, temperature dependent Hall coefficient K(T), mains voltage U, mains current I, and conversion coefficients k.sub.1, k.sub.2.
Because of the temperature dependent coefficient K(T) the Hall sensor as such is not appropriate for application in wattmeters for which high-accuracy performance in a broad temperature interval is required.
There are known various methods of temperature compensation, mostly using thermistors. A thermistor series-connection to the Hall element is described in the Japanese patent JP 57 121288. A deficiency of this compensation method is that the thermistor temperature dependence is different from the one of the Hall element, for which reason an exact compensation can hardly be accomplished.
Further, the temperature dependence of the Hall voltage is compensated with a separate measuring circuit, in which the temperature is sensed by a thermistor and a correction voltage is generated, which is added to the Hall voltage (U.S. Pat. No. 4,327,416). This is a demanding and expensive system since two separate measuring circuits and a large capacity storage unit are required.
The actual Hall element output voltage is very low, often below the level of disturbing voltages such as the Hall voltage originating from feeding current induction or from the quadratic term in K(T). With known voltage-frequency converters and A/D converters no signals with a level below the level of disturbing voltages can be measured.
It has not been possible to make high-accuracy wattmeters comprising Hall elements because of an asymmetric geometry and of a piezoresistive effect of Hall elements. By these two disturbing influences an alternating component is generated following the feeding current behaviour. In known wattmeters (e.g. JP No. 58 154263 and DE No. 30 01 772) this disturbing voltage is reduced by a special geometric disposition of differently oriented Hall elements within the Hall sensor. Voltage terminals of Hall elements are either series- or parallel-interconnected. The disturbing voltage is reduced, yet the interconnected sensor elements influence each other. On the other hand, there is known a circuit, in which to separately tapped voltages of particular elements (patent DE No. 30 26 226) offset voltages of differential transistor circuits are superposed.